In many cases, electrochemical storage cells are combined into so-called “monobloc batteries”, also referred to as “monoblocs”. These generally comprise physically identical rechargeable battery cells, which have the same capacity and are arranged in a common monobloc box, being connected in series. Their combination in a common housing firstly makes them easier to handle while, secondly, they can be produced at a lower cost and have a higher weight-related and volume-related energy density, due to the saving of material and due to the reduction in the number of individual parts.
Monobloc batteries such as these are normal, for example, in lead-acid rechargeable batteries. Monobloc batteries such as these are known for stationary applications, for example, in telecommunications and for emergency power supply facilities. In the case of lead-acid rechargeable batteries such as those used as starter batteries for vehicles, six identical cells are frequently connected in series, thus resulting in a block with a rated voltage of 12V. However, for stationary battery systems, monobloc batteries with only 2, 3, 5 or else a different number of cells connected in series are also normal.
However, monobloc batteries are used not only for lead-acid rechargeable batteries, but also for alkaline rechargeable batteries with the electrochemical systems nickel/cadmium, nickel/metal hydride, nickel/iron as well as for lithium-ion rechargeable batteries.
Depending on the nature of the electrochemical system, a certain amount of gases can be produced in rechargeable batteries when being used correctly and in extreme operating states. Such gases are emitted to the environment through an opening provided in the cell. These gases are primarily oxygen or hydrogen, or a mixture of these two gases, although the gas mixture may also contain other substances. The amount of gas developed depends on the type of electrochemical system, the nature and the operating conditions.
In the case of so-called “open rechargeable battery systems”, it is normal to produce a connection which is always open from the interior of the cell to the environmental air, into which precipitation devices such as baffle plates, frits or the like can be installed to reduce the emission of liquid droplets.
Sealed rechargeable batteries, on the other hand, are designed such that no gases, or only very small amounts of gases, are developed in the cell interior during normal operating conditions. For this reason, no excess pressure, or only a very small excess pressure, is built up inside the cells, and the connection between the cell interior and the environment is generally sealed by a nonreturn valve. This opens only when a certain excess pressure has occurred in the cell interior, and closes off the cell interior from the air surrounding the cell once again, in a sealed manner, once the excess pressure has been compensated for.
The gases formed in the interior of the cell may be combustible, depending on the nature and operating conditions, and in the case of a disadvantageous mixture ratio of oxygen and hydrogen, may even be directly explosive. For this reason, in many cases, the gas leaving individual cells is not emitted directly to the environment, but is collected in a gas collecting line which connects degassing openings of the individual cells in the monobloc battery, and is emitted from the monobloc battery to the environment from a single opening (or else, for redundancy reasons, from two openings). In order to avoid the collection of explosive gas mixtures, in, for example, a motor vehicle in which the starter battery is arranged in an area that is closed in a relatively sealed manner, a hose may be connected to this opening or to these openings, through which the gases which are developed are passed to the open air.
This procedure is not only normal for open and closed rechargeable batteries, but is also used for sealed rechargeable batteries.
The gas collecting lines are used to safely dissipate combustible gases or gas mixtures which may be developed from the area surrounding the monobloc rechargeable battery. In this case, it is very important to safely avoid detonation of the gases. The critical factor in this case is, in particular, flashbacks from flames or sparks outside the monobloc battery through the gas collecting line into the interior of the monobloc rechargeable battery. This is normally avoided by the installation of frits, composed of porous sintered plastic or gas, at the outlet of the gas collecting line to the environment.
These gas collecting lines are normally dry, that is to say, they are not wetted with electrolyte from the cells in the monobloc battery. The wall of the gas collecting lines is, thus, electrically decoupled from the cells since the material of the monobloc battery is generally an electrical insulator, in the same way as that of the gas collecting lines. The inner wall of the gas collecting lines is, thus, at an undefined potential and there are no high field strengths resulting from the voltage of the rechargeable battery cells.
However, the situation changes if the inner wall of the gas collecting line is wetted by electrolyte from one or more of the cells. In this case, a greater or lesser extent of electrolytic coupling occurs to one or more cells. If coupling occurs to a number of the series-connected individual cells, then an ionic current flows via this electrolytic wetting film on the common gas collecting line, since the series-connected cells are actually at different potentials. This ionic current results in an irreversible material transfer from one cell to the other, and to a discharge between the cells affected and the cells located in between them.
Ionic currents through the common gas collecting line are, therefore, parasitic currents, and must as far as possible be prevented. This is done by means of a gas collecting line design which is as long as possible and avoids permeating wetting with electrolyte, not only for the supply line from the individual cells to the gas collecting line, but also through the gas collecting line being routed over as long a distance as possible.
The ionic current which may flow from one cell to the others through the gas collecting line that has been wetted with electrolyte can lead to electrolysis of the water contained in the electrolyte film. As a consequence, the electrolyte film may be interrupted at specific points, so that very high field strengths may occur locally. If the field strength is sufficient, a flashover can occur, which may cause detonation of any explosive gas mixture which may be present at this point. The precise level of these field strengths is highly random and virtually unpredictable, although it has been found from experience that, if the voltages are less than 30 volts, there is generally no risk of detonation of the gas mixture that is located in a gas collecting line.
Where higher voltage differences are possible between cells which are connected to a common gas collecting line, it is possible, on the other hand, for an explosive gas mixture to be detonated. This is described and explained in German Patent Specification 3,425,169. In order to overcome the risk of detonation, this Patent Specification describes a method in which alternately insulating and metallically conductive sections are arranged in the gas dissipation system. The metallically conductive sections are connected to one another by means of electronic switching elements, namely simply by non-reactive resistors or, in an improved embodiment, by zener diodes. This limits the voltage difference between adjacent metallically conductive sections.
Furthermore, FIG. 2 of DE-C 3,425,169 indicates the connection of that metallically conductive section which is closest to the positive pole being connected to the positive pole.
This method is particularly suitable for large battery systems, such as those which are used in submersible vehicles. In this case it is normal to collect charging gases that are developed in the individual cells in a common dissipation system. In this case, cells which may have voltage differences of several hundred volts between them are connected to one another. A similar situation occurs in large fuel cell units, whose exhaust gas lines are likewise connected, and where, once again, voltages of several hundred volts, may occur. However, the measures described in German Patent Specification 3,425,169 are comparatively complex, since they require sections of metallically conductive dissipation channels and electronic circuitry.
In monobloc batteries in which sufficiently few electrochemical cells are interconnected that it is impossible to reach a voltage that is sufficient for detonation of an explosive gas mixture, in any operating conditions, in a degassing system that is provided, there is no need for any special precautionary measures. This applies in particular to lead-acid rechargeable batteries with a rated voltage of 12V, such as those which are used in conventional present-day starter batteries in motor vehicles, which are in general not subjected to voltages of more than 18V, even in extreme conditions.
Concepts for novel power supply system networks for use in motor vehicles envisage that the currently normal operating voltage for the electrical components of about 14V will be raised to a new voltage level of about 42V. This application requires batteries whose rated voltage is about 36V. If these are lead-acid rechargeable batteries, then charging voltages of up to about 48V must be provided in this case.
International specialist committees are currently considering draft standards for the voltage level in such new vehicle power supply system networks, which envisage operating voltages of up to 48V with positive half-cycle peaks of up to 50V. Furthermore, in exceptional cases, voltage peaks of up to 50V are permissible (see K. Ehlers, H.-D. Hartmann, E. Meissner, Journal of Power Sources 95 (2001) 43).
Batteries for these new concepts for electrical power supplies for motor vehicles may be composed of a number of monobloc batteries with an appropriate rated voltage. When, for example, two monobloc batteries each having a rated voltage of 18V are used, then it would actually be possible to reach a voltage value that is critical for detonation, in the event of an asymmetric distribution of the voltage between the two monobloc batteries, when using a vehicle power supply system network voltage with a maximum of 58V.
If three monobloc batteries with a rated voltage of 12V each are connected in series, then voltages in an individual block of more than 30V can be achieved only with an extremely non-uniform voltage distribution, for example, in the event of a defect in one of the blocks. There is, thus, no need to be concerned about any risks resulting from detonation in a gas collecting line in the individual blocks in this case, provided that these systems of the individual blocks are not connected to one another.
However, if there is an aim to provide a battery for this vehicle power supply system network in a single monobloc battery, then the voltage in the gas collecting line in this monobloc battery will undoubtedly exceed the threshold values which are critical for detonation, if a single gas collecting line is provided, that is used for all the cells. The same is true of the gas collecting lines of a number of series-connected monobloc batteries, which each have only a low rated voltage, are connected, thus resulting in a gas dissipation system with an overall voltage which may be critically high overall.
It is also possible to provide a number of separate gas collecting lines with physically and electrically separated outlets to the environment in a monobloc battery, so that the voltage on each of them remains below the possibly critical value of 30V with the voltage conditions that can be assumed. However, this necessitates each of these outlets having its own frit to avoid backflashes from the exterior. However, this increases costs. If there is also an aim of fitting flexible tubes to the outlets of the rechargeable battery, to dissipate the gases that are developed further, then a number of outlets, with separate flexible tubes, must be provided. This considerably increases the risk of incorrect fitting, especially after replacement of the rechargeable battery.
It would, therefore, be advantageous to provide a simple and low-cost procedure in order to avoid the risk of detonation of the explosive gas mixtures in gas collecting lines in monobloc batteries, where the gas collecting lines connect cells to one another which can reach an overall voltage of more than 30 volts with respect to one another during operation, and which are used in particular in monobloc batteries with a comparatively low rated voltage (such as a rated voltage of 42V for a motor vehicle power supply system network).
This is equivalent to interconnections of a number of monobloc batteries with a relatively low rated voltage with their respective gas collecting lines connecting in series, in which case, although no critically high voltages occur in the gas collecting line of each individual monobloc battery, such voltages do, however, occur in the overall degassing system that results from the connection of the gas collecting lines.